- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)

Programs called by the key handler might replace symlinks instead of their
targets.

Old snprintf calls could have overflowed the buffers.

- Only load the thumbnails that are currently visible in the window
- Unload thumbnails that are leaving the visible area
- Much less memory needed, but scrolling is now slower
- This also unintentionally fixes issue #86

The function "free" performs input parameter validation.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/free.html

It is therefore not needed to check a passed pointer before this function call.
A corresponding update suggestion was generated by the software "Coccinelle"
from the following semantic patch approach.
http://coccinelle.lip6.fr/

@Remove_unnecessary_pointer_checks1@
expression x;
@@
-if (x != \(0 \| NULL\))
    free(x);

@Remove_unnecessary_pointer_checks2@
expression x;
@@
-if (x != \(0 \| NULL\)) {
    free(x);
    x = \(0 \| NULL\);
-}

@Remove_unnecessary_pointer_checks3@
expression a, b;
@@
-if (a != \(0 \| NULL\) && b != \(0 \| NULL\))
+if (a)
    free(b);

@Remove_unnecessary_pointer_checks4@
expression a, b;
@@
-if (a != \(0 \| NULL\) && b != \(0 \| NULL\)) {
+if (a) {
    free(b);
    b = \(0 \| NULL\);
 }

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>

- Buffer for window content is bigger than the window, minimizes artifacts
  when window is resized
- Back to using XSetWindowBackgroundPixmap() instead of XCopyArea(),
  no need to handle exposure events; X server can show gray background directly
  after resize event before sxiv redraws the window contents

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

Issue described here:
https://bbs.archlinux.org/viewtopic.php?pid=1117294#p1117294